The present invention relates generally to semiconductor processing, and more particularly, to methods for producing high quality crystalline material over a buried insulating layer in a semiconductor substrate.
A class of semiconductor structures, known as silicon-on-insulator (SOI) or semiconductor-on-insulator structures, include a thin superficial silicon layer over a buried insulating layer. SOI structures are widely utilized for construction of electronic devices. For example, such structures can be employed to produce semiconductor devices, such as VLSI devices, micro-electro-mechanical systems (MEMS), and optical devices. One method of producing an SOI structure, known by the acronym SIMOX (separation by implanted oxygen) forms a buried oxide layer (BOX) in a semiconductor substrate by implanting oxygen ions into the substrate followed by a high temperature annealing step. The insulating layer provides electrical isolation of devices that are built in the superficial silicon layer.
The implantation step of a SIMOX process typically generates defects in the upper silicon layer even when the substrate is held at an elevated temperature during the implantation step to induce dynamic annealing of damage. The doses of oxygen ions that are utilized in SIMOX processes can cause detectable strain in the upper layer lattice that can in turn generate defects, such as dislocations and/or stacking faults.
Accordingly, there exists a need for better SIMOX processing techniques to form a buried insulating layer in a substrate such that the defect density is reduced in the layer above the buried layer.
Such SOI structures having upper silicon layers with substantially fewer dislocations would address a long-felt need in the art.
Methods of producing buried insulating layers in semiconductor substrates are disclosed whereby a dose of selected ions is implanted into a substrate to form a buried precursor layer below an upper layer of the substrate, followed by a thermal treatment including oxidation of the substrate in an atmosphere having a selected oxygen partial pressure to form an oxide surface layer. The oxidation is performed at a temperature and for a time duration such that the formation of the oxide layer causes the movement of a controlled number of atoms of the substrate material from the interface of the newly formed oxide layer into the upper region of the substrate. The injection of the controlled number of atoms prior to the onset of precipitation results in reduction of strain in the lattice of the upper layer.
The oxidation temperature can be selected, for example, to be in a range of approximately 750xc2x0 C. to 1150xc2x0 C., and the oxidation duration can be selected to be in a range of approximately 1 to 120 minutes. The injection of a controlled number of the atoms of the substrate into the upper layer advantageously reduces strain in the upper layer. The method also includes a step of annealing the substrate to form the insulating layer within the precursor layer.
The ion implantation step of a method of the invention can be performed by utilizing a variety of different techniques. For example, a selected dose of ions can be implanted in a substrate by exposing the substrate to a beam of the ions having a selected energy. The ion energy can be, for example, in a range of about 20 keV to 450 keV. Alternatively, a plasma immersion technique can be employed for implanting a selected dose of ions in the substrate. The dose of implanted ions can be, for example, in a range of approximately 1xc3x971014 to 2xc3x971018 ions/cm2.
The method of the invention can be practiced on different substrates and by employing different implanted ions. For example, the substrate can be silicon (Si), silicon carbide (SiC), or SiGe. Further, the implanted ions can be selected to be oxygen or nitrogen ions. For example, a dose of approximately 2xc3x971017 ions/cm2 of oxygen ions at an energy of about 65 keV can be implanted in a silicon substrate. The implantation step is preferably performed while maintaining the substrate at an elevated temperature, for example, at a temperature in a range of approximately 200xc2x0 C. to 700 xc2x0 C.
According to a related aspect of the invention, the method calls for performing the annealing step while maintaining the substrate temperature in a range of approximately 1300xc2x0 C. to a temperature below a melting temperature of the substrate. The annealing step can be performed for a time duration of approximately a few hours. The annealing time duration can be, for example, in a range of approximately 4 hours to 30 hours. The annealing step can be performed in an inert atmosphere. For example, the annealing step can be performed in an inert atmosphere having a trace amount of oxygen (e.g., less than 1% oxygen).
In another aspect, the method of the invention calls for forming a superficial overlayer on a surface of the substrate prior to the implantation step to control a depth of ion implantation in the substrate. The superficial layer can be formed, for example, by depositing a film on the substrate. The deposited film can include, for example, SiO2, amorphous silicon, or Poly-Si. The film can also include a multi-layered structure.
In a related aspect, the method of the invention can optionally remove the superficial overlayer before performing the oxidation step if oxygen atoms do not readily diffuse through the superficial overlayer to oxidize the interface of this layer and the substrate to cause injection of substrate atoms into the upper layer. For example, when a film of silicon nitride (Si3N4) is utilized as the superficial overlayer, the method calls for removing this film before performing the oxidation step.
Another aspect of the method of the invention ramps up the temperature of the substrate after the implantation step from ambient (e.g., 25xc2x0 C.) to an intermediate value (e.g., about 600xc2x0 C.) at a first selected rate (e.g., approximately 40xc2x0 C./min), and further ramps up the temperature from the intermediate value to the oxidizing temperature (e.g., about 1000xc2x0 C.) at a second selected rate (e.g., approximately 10xc2x0 C./min).
In yet another aspect, the method of the invention calls for disposing a patterned mask on the substrate before the implantation step in order to implant ions in selective portions of the substrate, thus forming a buried insulating layer in these portions.
In another aspect, the oxidation step of the method of the invention is performed while the substrate temperature is ramped up from an initial value to a final value. The initial and final temperatures, and the ramp rate are selected such that the formation of the oxide layer causes injection of a controlled number of atoms of the substrate from a region proximate to an interface between the oxide layer and the substrate into the upper layer. The injection of the controlled number of atoms advantageously reduces strain in the upper layer.
Illustrative embodiments of the invention will be described below with reference to the following drawings.